Increasingly, electronic devices are designed to operate from ever smaller power supply voltages. Smaller supply voltages are designed to minimize leakage currents and other undesirable effects that increase at smaller transistor sizes (e.g., 0.18 microns). Smaller supply voltages also save power, which is an especially important consideration in portable devices that operate from a battery. For example, current cellular telephones and other portable applications operate from a +1.8 volt power supply rail.
However, problems are encountered as power supply voltage levels become ever smaller. In applications where a given amount of power is required, the trend toward smaller supply voltages means that the current requirement must be increased to offset the reduced power supply voltage. For example, in a cellular telephone, a given amount of power is required to drive the speaker to an audible level. To maintain this power level, the current driving the speaker must increase as the output voltage decreases. The impedance of the speaker becomes smaller with the supply voltage in order to maintain the output power due to a larger current.
But difficulties are encountered when trying to combine a rail-to-rail output voltage range (0 to 1.8 volts) with a large output current (e.g., 60 mA). In order to drive a large current with a small drain-source voltage, VDS, as is the case with the output transistors of a speaker driver, large W/L (channel width (W) to channel length (L) ratio) values are needed. As a result, the quiescent gate-to-source voltages, VGS, of these transistors tend to become very small in order to keep the quiescent current, Iq, acceptably small.
However, if a Class-AB push-pull operational amplifier drives the speaker, the VGS of the push-pull transistors of the output stage should be at least larger than one saturation voltage VDS,SAT of the output transistors of the previous stage in order to maintain their high output resistance and thus the voltage gain of the first stage. Therefore, large output transistors become very difficult (or even impossible) to bias in a conventional Class-AB operational amplifier with an acceptably small quiescent current, Iq.
One solution to this problem that has been proposed provides for a Class-AB+B operational amplifier that is able to operate from small power supply voltage levels while still driving large output currents and having a small quiescent current. This solution divides the output transistors into relatively large Class-B output drivers in parallel with relatively small Class-AB transistors. The small Class-AB transistors have a minimum quiescent transconductance, whereas the Class-B current booster transistors are inactive in the quiescent state and only deliver large currents for output voltages approaching the power supply rails. The Class-B current booster stage has minimum impact on the Class-AB amplifier stage and can be adjusted independently whenever larger currents are needed in future applications.
However, with this approach, level shifters impede the gate-to-source voltages of the Class-B output transistors from becoming as large as the gate-to-source voltages of the Class-AB output transistors. As a result, the output voltage of the operational amplifier clips earlier than in comparable Class-AB stages with the same output transistors. Thus, the drive strength of the Class-B stage is not fully exploited.
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